I'm a Nigerian with a 3 month Schengen visa to Athens, Greece double entry. My flight is Air France route Lagos-France-Greece. Can I be stamped in at France at a port of entry before my connecting flight to Greece? Secondly, can I be allowed to shop in France in an hour time to meet my connecting flight whereas I have 2 hours awaiting on transit before my connecting flight to Athens? Finally, can I have a stop over in France to continue via local air to Greece the next day as I want to see a friend before leaving France?
When flying into the Schengen zone, the first airport you visit is where you will clear immigration (have your Passport stamped). Your checked luggage continues to your final destination where you clear customs. Your luggage is differentiated from local Schengen traffic by the lack of green stripes on the bag tags (green stripes are printed on the tags only for trips originating in the EU, so customs agents know to look for them).
During your layover at Paris's Charles de Gaulle airport, you can certainly shop inside the airport. However, you probably cannot purchase from the duty free shops. If you only have a 2 hour connection in Paris, you don't have enough time to leave the airport to shop elsewhere.
Most airlines will allow you to add a stopover to your trip based on the ticket's fare rules; in some cases, a stopover may require an additional fee, which is rolled into the ticket price. Just talk to the airline to request a stopover!
Edited 7/2/2016 to reflect updated green-strip printing information from comments.
With a Schengen visa, you can enter/exit any country within the Schengen area, no restrictions. It does not matter which country issued the visa.
With a longer-stay national visa (e.g. a Greek student visa), rules might be different.
You should check if a Nigerian citizens need a visa to transit France on their way to Greece. Or for that matter, does a Nigerian citizen need a visa to visit France. You can check this at http://www.iatatravelcentre.com/passport-visa-health-travel-document-requirements.htm.